In judgement of the operating state of a plant, etc., a technology wherein output values of various sensors installed in the plant produced during normal operation are stored beforehand as reference values, sensor output values read during an actual operation are compared with these reference values in order to judge whether the sensor outputs are normal or abnormal, and results are used to judge whether the operation of the plant is normal or abnormal is widely known as a limit check method. However, since a transient state and a steady state exist in operations, it is not possible to judge abnormalities accurately if the same reference values are used for both these states. Japanese Laid-open Patent Application No. 2-114399 therefore disclosed a technology which is designed to diagnose abnormalities accurately regardless of the operating state, by automatically changing reference values in accordance with the states of plant process values.
The case where a plant is a generator and the abovementioned conventional technology is applied in abnormality diagnosis will now be considered. In FIG. 3, 1 is a generator, 2 is a voltage sensor for the generator, 3 is a rotational speed sensor for the generator, 4 indicates power supply wiring and 5 indicates various loads which receive supply of power.
With this configuration, since there is a limit (capacity) associated with the ability of the generator to supply power, if there are many loads, the voltage tends to drop and becomes insufficient for the rated voltages of the loads. In such a case, depending on the amount of power that has to be supplied to the loads 5, it can happen that the output value of, e.g., the voltage sensor is less than the normal reference value even though the generator is normal, regardless of whether the operating state is a transient state or a steady state.
In the above-described configuration, since the output values of the voltage sensor 2 and the rotational speed sensor 3 ought to be in a relation of correspondence, sensor abnormalities can be detected by comparing correspondence states of the output values of the two sensors. In the system of FIG. 3, it is assumed that a relation as indicated by formula (1) below and a sensor abnormality judgement rule (a) below apply to the output values of the two sensors. EQU V=.alpha..times.R+.beta. (1) EQU if r1.ltoreq.R.ltoreq.r2 then v1.ltoreq.V.ltoreq.v2 (a)
V: voltage PA1 R: rotational speed PA1 .alpha., .beta.: constants
In this case, regardless of whether the operation is in a transient state or in a steady state, if the total amount of power that has to be supplied to the loads 5 exceeds the supply upper limit of the generator 1, then it is judged by the rule (a) that a sensor abnormality has occurred on the grounds that "although the rotational speed is between r1 and r2, the voltage value is less than v1.", even though the sensors are normal.
Thus, with the above-described conventional technology, there is the problem that, since the states of system load are not taken into consideration, it is not possible to judge system or sensor abnormalities accurately.
With the foregoing in view, it is an object of the present invention to provide a measurement value judging method which makes it possible to gain an accurate apperception of system operating states and to make accurate system and sensor abnormality judgement.